1. Field of the Invention
The present invention relates to a drive device capable of reproducing data from a recording medium.
2. Description of the Related Art
In drive devices for reproducing data from a recording medium such as an optical disk or a magneto-optical disk, when a certain sector (unit of data recorded on a recording medium) is read, there is a possibility that a decoding error occurs for that sector. One possible cause for that is an inadequate level of laser power used to record data into that sector.
In general optical disk systems, the recording laser power is set to an adequate level depending on a medium used and the conditions of the drive device. In order to determine the optimum level of the recording laser power, calibration is performed when data is recorded.
However, in practice, data is not always recorded with optimum recording laser power even when calibration is performed.
For example, calibration is performed as follows. First, test recording is performed upon a certain region of a medium, and then the recorded data is reproduced. If the test recording reveals that the data was not recorded adequately, the recording laser power is adjusted. However, if the recording sensitivity of regions where data is actually recorded is different from that of the region used in the calibration, the recording laser power determined by the calibration is not optimum in the actual recording operation.
A similar problem also occurs when calibration is performed in an incorrect manner for some reason, or when the recording laser power is not correctly set to the level determined by the calibration.
In some cases, when data is recorded in a certain sector, the data recorded in that sector is immediately reproduced to check whether the data has been recorded correctly. This operation is called a write and verify operation. If it is determined in a verify process that data was not recorded adequately due to an inadequate level of the recording laser power, a recording operation may be retried after setting the recording laser power to an adequate level. However, in some drive devices, the write and verify operation is not performed for all data sectors. Besides, there are many drive devices which do not perform the write and verify process.
For the above reasons, the recording laser power is not necessarily adequate for all data to be reproduced. As a result, a reproduction error can occur owing to an inadequate recording power level.
If a drive device has a high reading ability, there is a possibility that the drive device can correctly reproduce data which has been recorded with inadequate recording laser power. For example, drive devices employing the Viterbi decoding method, which is now practically used in the art, have considerably higher reading ability than those based on the bit-by-bit reading technique.
In the Viterbi decoding method, decoding is performed on the basis of amplitude reference values as will be described in detail later. A method has been developed to adaptively modify the amplitude reference values depending on the signal being reproduced. This method allows a further improvement in the reading ability.
That is, this method allows data to be correctly reproduced by adaptively modifying the amplitude reference values depending on the RF signal being reproduced, even for data recorded with inadequate recording laser power.
However, if the deviation of the recording laser power from the optimum value is too great, the adaptation of the amplitude reference values is impossible, and thus it becomes impossible to correctly reproduce data. More specifically, in a region near the leading end of a sector being reproduced, the amplitude reference values have not been well adapted yet, and thus there is a rather high probability that the amplitude reference values deviate greatly from the values optimum for the RF signal being reproduced. Such a deviation often results in a sector decoding error.
In view of the above problems, it is an object of the present invention to provide a drive device employing the Viterbi decoding method and capable of correctly reproducing even data recorded with inadequate recording laser power.
According to an aspect of the present invention, there is provided a drive device comprising: head means for reading a data signal recorded on a recording medium by illuminating the recording medium with a laser beam; decoding means for performing a Viterbi decoding process and a predetermined decoding process upon the data signal read by the head means thereby obtaining reproduced data; determining means for determining, from the data signal read by the head means, whether or not the recording laser power used to record the data signal is adequate; and control means for controlling a reproducing operation such that when the decoding means fails to correctly reproduce data from the data signal read by the head means (that is, when a decoding error occurs), if the determining means determines that the recording laser power associated with that data signal is inadequate, the reproducing operation is retried after changing an amplitude reference value used in the Viterbi decoding process.
That is, when a decoding error occurs, if the determining means determines that the recording laser power is inadequate, the retrying of the reproducing operation is performed after changing the amplitude reference value so that even the data recorded with inadequate recording laser power can be successfully reproduced.
Preferably, the determining means calculates the degree of asymmetry of the waveform of the data signal read by the head means and determines the adequacy of the recording laser power on the basis of the degree of asymmetry.
More specifically, the determining means preferably determines the adequacy of the recording laser power depending on whether or not the calculated degree of asymmetry is within a predetermined range.
Because there is a correlation between the degree of asymmetry of the reproduced RF signal and the recording laser power, the recording laser power can be estimated on the basis of the degree of asymmetry, and the retrying operation can be performed in a proper fashion depending on the estimated recording laser power (by adaptively changing the amplitude reference value depending on the recording laser power).
Various types of recordable disk media are known in the art. One type of such a disk medium is a WORM (write-once, read-many-times) disk. The WORM disk can be further categorized into an ablative-type WORM disk in which embossed pits is formed on a disk by means of illumination of a laser beam, a phase transition disk, and an alloy-type disk in which pits are formed such that they have reflectance different from that of non-pit portions.
Another type of recordable disk medium is a rewritable disk medium which can be further categories into a magneto-optical (MO) disk in which magnetic pits are formed, and a disk using phase transitions such as a DVD-RAM and a DVD-RW.
The disk media described above have a correlation between recording laser power and the degree of asymmetry of a reproduced data signal, except for the alloy-type WORM disk. Therefore, it is possible to determine, on the basis of the degree of asymmetry of the reproduced data signal, whether or not the recording laser power is adequate.
Preferably, the control means controls the reproducing operation such that when the operation of reproducing data from a region on the recording medium is started immediately after completion of reproducing data from a previous region on the recording medium, the current amplitude reference value is used in the Viterbi decoding process. The region (sector) following the previous region is not necessarily a region which is physically adjacent to the previous region. Herein, the following region refers to a region which is reproduced after the previous region.
A plurality of sectors recorded in one recording operation are very likely to have been recorded with the same level of recording laser power. Therefore, when the recording laser power of a certain sector is determined to be inadequate and an amplitude reference value is changed, if the changed amplitude reference value is employed in the operation of reproducing the next sector, the probability of successfully reproducing that sector is increased.